emacs: Define a common shared keymap for all of notmuch

This defines a single, currently empty keymap that all other notmuch
mode maps inherit from.

emacs/notmuch-hello.el

@@ -515,8 +515,14 @@ Such a list can be computed with `notmuch-hello-query-counts'."
 
 
 (defvar notmuch-hello-mode-map
-  (let ((map (make-sparse-keymap)))
-    (set-keymap-parent map widget-keymap)
+  (let ((map (if (fboundp 'make-composed-keymap)
+		 ;; Inherit both widget-keymap and notmuch-common-keymap
+		 (make-composed-keymap widget-keymap)
+	       ;; Before Emacs 24, keymaps didn't support multiple
+	       ;; inheritance,, so just copy the widget keymap since
+	       ;; it's unlikely to change.
+	       (copy-keymap widget-keymap))))
+    (set-keymap-parent map notmuch-common-keymap)
     (define-key map "v" (lambda () "Display the notmuch version" (interactive)
 			  (message "notmuch version %s" (notmuch-version))))
     (define-key map "?" 'notmuch-help)

emacs/notmuch-lib.el

@@ -128,6 +128,11 @@ For example, if you wanted to remove an \"inbox\" tag and add an
   :group 'notmuch-search
   :group 'notmuch-show)
 
+(defvar notmuch-common-keymap
+  (let ((map (make-sparse-keymap)))
+    map)
+  "Keymap shared by all notmuch modes.")
+
 ;; By default clicking on a button does not select the window
 ;; containing the button (as opposed to clicking on a widget which
 ;; does). This means that the button action is then executed in the

emacs/notmuch-show.el

@@ -1243,6 +1243,7 @@ reset based on the original query."
 
 (defvar notmuch-show-mode-map
       (let ((map (make-sparse-keymap)))
+	(set-keymap-parent map notmuch-common-keymap)
 	(define-key map "?" 'notmuch-help)
 	(define-key map "q" 'notmuch-kill-this-buffer)
 	(define-key map (kbd "<C-tab>") 'widget-backward)

emacs/notmuch.el

@@ -198,6 +198,7 @@ first line of documentation for the bound function."
 
 (defvar notmuch-search-mode-map
   (let ((map (make-sparse-keymap)))
+    (set-keymap-parent map notmuch-common-keymap)
     (define-key map "?" 'notmuch-help)
     (define-key map "q" 'notmuch-kill-this-buffer)
     (define-key map "x" 'notmuch-kill-this-buffer)